Noise generated by jet aircraft engines during takeoff and landing is a serious environmental problem and a matter of public concern in most parts of the world. Jet aircraft engines emit great quantities of high velocity gases at their exit nozzles and it is, in large measure, the shearing forces between the high velocity gases and the ambient air that produce a significant component of the high levels of noise that many find objectionable. Because of the adverse impact noise has on the environment, many countries have imposed increasingly strict noise reduction criteria on aircraft. In the United States, the Federal Aviation Administration has imposed strict noise reduction requirements which will place strong operating restrictions on aircraft that are currently in use. These restrictions range from financial penalties and schedule restrictions to an outright ban on the use of the aircraft. An effective and efficient noise reduction solution is necessary since these restrictions would severely cut short the useful life for certain types of aircraft that commercial airlines are currently using. For background information related to noise reduction systems for jet engines, reference may be made to the following U.S. Pat. Nos.: 3,710,890; 4,077,206; 4,117,671; 4,302,934; 4,401,269; 4,501,393; 4,909,346; 5,060,471; 5,167,118; and 5,440,875.
Sound is caused by pressure waves in the air, set in motion by a source. The level of pressure generated determines the intensity of the sound and can be measured by instrumentation. The annoyance of sound to a listener is determined by the intensity of the sound and the duration of that sound experienced by the listener. In the turbofan engine industry, the effect of noise on humans is expressed in terms of an effective perceived noise intensity level, EPNdB, based on the bel unit system for noise intensity.
Generally speaking, the jet noise generated by turbofan engines is normally dominated by two sources: the fan or bypass flow and the primary flow. These two sources are concentric components that flow in axial streams out of the engine's exhaust nozzle, to produce useful thrust.
Turbofan engines are categorized as either high bypass ratio or low bypass ratio based on the ratio of bypass flow to core flow. As air enters the front of the jet engine it passes through the fan and is split between the primary flow and bypass flow. The primary flow first enters the low pressure compressor then the high pressure compressor. The air is then mixed with fuel and the mixture is burned thereby increasing its pressure and temperature and is passed into the high pressure turbine and low pressure turbine where the energy is converted to work to turn the fan and compressor and to useful thrust in the form of exhaust. The bypass flow is created by the fan and passes outside the core of the engine through a duct and is exhausted as useful thrust. Generally, turbofan engines having a bypass ratio of two or less are categorized as low bypass ratio engines. In low bypass ratio engines the core flow and the bypass flow enter the exhaust at the nearly the same pressure but not at the same velocity or temperature. It is a characteristic of a turbofan engine that noise increases with increased relative difference in velocity between fan and core flows and that velocity is proportional to temperature. In a typical turbofan engine with a bypass ratio approximately equal to two, at a given power setting, the flow temperature and velocity in the primary duct are typically on the order of 940.degree. F. and 861 ft/s respectively, and the fan duct flow temperature and velocity for the same power setting are 220.degree. F. and 442 ft/s. Noise from the jet exhaust of the core flow is generated in regions behind the engine by the turbulent mixing of the core and fan exhaust streams and also where the high velocity jet exhaust mixes with the ambient air.
Experience in the turbofan industry has proven that the mixing of the two air flows before they exit the exhaust nozzle is beneficial in reducing perceived noise levels. As defined by the bypass ratio, there is a larger volume of low velocity, low temperature fan flow than the high velocity, high temperature primary flow and mixing the two flows serves to lower the overall exhaust velocity of the mixed air and therefore, lowers the overall perceived noise level. In a typical low bypass ratio turbofan engine, unless they are mixed, the two flows will remain substantially distinct as they exit the exhaust nozzle of the engine and the perceived noise level will remain high. In general, a more homogeneous mixed flow, prior to exiting the engine's exhaust nozzle, will result in a lower overall temperature and velocity of the mixed the flow, which in turn results in a lower perceived noise level.
The object of forced mixing within the engine is to lower the overall velocity of the flow before combining with the ambient air to decrease the noise produced by the exhaust jet mixing with the ambient air. Many schemes have been employed in an attempt to optimize forced mixing through a trial and error evolution that has resulted in various geometries, one in particular being the lobed mixer. Most commercial applications of lobed mixers achieve typical noise reductions in the range of 3.5 to 4.5 EPNdB. Since the early 1960's forced mixing, as described in U.S. Pat. No. 3,027,710, has been used to reduce the noise produced by low bypass ratio turbofan jet engines. Such forced mixing for noise reduction purposes is commonly accomplished through the use of a lobed mixer attached to the engine near the turbine exit, immediately upstream from the exhaust nozzle. The lobed mixer typically has a number of circumferentially spaced lobes of increasing radial height in the axial downstream direction, arranged in a fashion whereby chutes are formed between the lobes. The fan air is directed through chutes between the lobes on the outer surface of the mixer and the primary core flows through chutes defined by the lobes on the inside surface of the mixer. The chutes and lobes operate to mix hot, high velocity core flow with a larger volume of cooler, low velocity fan air. Due to less than complete mixing in prior art lobed mixers such mixers produce a number of discrete segments of mixed air downstream of the mixer. The number of segments produced is equal to the number of lobes on the mixer and the segments are essentially similar in temperature profile. However, one problem with such lobed mixers is that the temperature varies in the radial direction within each downstream segment such that local hot spots are defined near the center of each lobe location. These local hot spots are made up of hot, high velocity gases and are caused by incomplete mixing of the two streams within the exhaust duct. With prior art lobed mixers, it is virtually impossible to eliminate such hot spots without reducing the effectiveness of the lobed mixer as a sound suppression device.
Forced mixing has also been used for other purposes such as in U.S. Pat. No. 3,048,376, Howald et al. In the Howald patent the mixer is used to achieve a mix of the fan flow and core flow before the gas enters an afterburner to realize a more efficient combustion in the afterburner itself. This type of application differs greatly from the use of a mixer in commercial aircraft for noise reduction purposes. First, the Howald et al mixer is positioned just in front of the afterburner inside the engine duct, a substantial distance upstream of the exhaust nozzle, and is an integral part of the combustion process. Second, afterburners are typically used on military aircraft where noise levels are not a primary concern. Third, the Howald et al mixer has secondary lobes running the full length of the mixer accounting for a large increase in surface area, resulting in pressure losses which may be unacceptably high from a commercial perspective. Lastly, since the mixer is upstream of the afterburner, the mixed gas is subsequently infused with fuel and ignited in the afterburner, further accelerating the gas which generates a tremendous level of noise when it finally exits the exhaust nozzle of the engine, because the gas is much hotter and flowing at a much higher velocity than in an engine without an afterburner, thereby obviating the mixer as a noise reduction device.
The practice of forced mixing for noise reduction purposes does not come without cost. Frequently, increased mixing for noise reduction purposes comes at the expense of increased flow losses which result in performance losses and reduced fuel efficiencies for the same power level settings. For example, in a commercial aircraft, a one percent loss in performance could result in a decrease of over twenty five nautical miles in maximum range and an increase of one half of a percent in operating costs.
The objective in a lobed mixer is to achieve the highest level of uniform mixing of the two flows prior to exiting the exhaust nozzle. A common concept within the industry is that the more lobes a mixer has, the better the uniformity of the mixed gas and the greater the reduction in perceived noise. This concept attempts to produce a uniform mix circumferentially about the engine centerline at the exit plane of the exhaust nozzle. However, when the number of lobes is increased, so too is the surface area over which the gas must flow prior to exiting the exhaust nozzle. When the area is increased the pressure losses due to drag across these surfaces increase. The converse is also true. The fewer lobes a mixer design has, the lower pressure losses, the lower the uniformity of mixing, and the lower the effectiveness in sound suppression. Heretofore the process of designing an effective lobed mixer has been to optimize the number of lobes to achieve a satisfactory trade-off between sound suppression and pressure losses.